Security documents such as identification cards, passport's, driver's licenses, health cards, etc. typically include one or more printed images which are used to identify the cardholder or other entity associated with the security document. For example, a security document in the form of an identification card, a driver's license or personal identification sheet of a passport may have a thermoplastic substrate and bear a printed photograph of the person to whom the card, license or passport was issued (referred to as a personalization image) to identify and verify a person holding the security document. This photograph may, for example, be printed onto the thermoplastic substrate by means of laser marking, whereby a laser sensitive thermoplastic substrate is marked by applying a laser beam to it which causes it to discolour (i.e. mark), or by ink jet printing using an ultraviolet (UV) curable ink.
It is desirable that personalization images (i.e. images used to authenticate the identity of a cardholder) applied to security documents be resistant to tampering by forgers, such as by replacing an authentic, personalized image with a non-authentic image. Typically, security documents display such a personalized image within a transparent window formed in an opacified substrate of the document. To forge such a security document a forger may choose to alter such image by cutting out the window and replacing it with another displaying a different image. Consequently, there is a need to protect the window of a security document against fraudulent removal in order to protect against such tampering and improve security.